During exploration of a geological formation bearing oil and/or gas (hydrocarbon), and during development and management of extraction of hydrocarbon therefrom, it is important to be able to obtain representative samples of hydrocarbon from the formation. It is known to do this using a down hole wireline formation testers or sampling tools such as the Modular Dynamics Formation Tester (MDT) tool of Schlumberger. For a description of such equipment see Badry, R., Head, E., Morris, C. and Traboulay. I., “New wireline formation tester techniques and applications”, Trans. SPWLA 34th Ann. Logging Symp., Calgary, June 1993, paper ZZ, and Schlumberger, Wireline Formation Testing and Sampling, pp. 10-1 to 10-25, Schlumberger Wireline and Testing, Houston (1996). The MDT tool allows samples of hydrocarbon to be captured and maintained at reservoir pressure in sealed containers. The samples are recovered at the surface and analysed to determine their composition (in terms of the relative amounts of different hydrocarbons) and their phase (pressure-volume-temperature or PVT) behaviour.
The acquisition of representative hydrocarbon samples from permeable formations can be impeded by several problems.
A major difficulty that commonly occurs when sampling is contamination by invasion of significant quantities of drilling fluid. This is particularly the case when using oil-based drilling fluids (also known as oil-based muds or OBMs). The base oil in OBMs usually consists of refined mineral oil, unrefined diesel oil or so-called synthetic oil such as poly(alphaolefins) or esters derived from vegetable oils. The OBM further comprises suspended solids in the form of weighting agent (commonly barite), fluid loss control agent (commonly clay) and possibly also drilled solids generated during drilling. The continuous liquid component of OBMs (the filtrate) is completely miscible with the formation hydrocarbons, and it is difficult to distinguish the filtrate from the formation hydrocarbons. This frequently results in samples of formation hydrocarbon contaminated with filtrate being collected. Such contamination results in inaccurate composition and phase behaviour information being obtained.
Another consideration is the length of sampling time. While sampling times may be prolonged with the aim of initially pumping out drilling fluid and contaminated formation hydrocarbon, thus reducing the likelihood of obtaining contaminated samples, longer sampling times increase the risk of the sampling tool and/or cable sticking in the hole. The sticking can be caused by the difference in the pressures in the drilling fluid column and permeable formations (so-called differential sticking) or by the condition or the geometry of the hole (key seating, swelling/collapsing shale sections etc). There is therefore a direct conflict between the need for extended sampling times to reduce sample contamination and the need for short sampling times to reduce the risk of sticking.